Conventionally, the amount of data handled within a particular group, such as a business, university, municipality, etc., continues to increase. Adverse effects of increases in data volume at a business, for example, include an excessive amount of file servers, independent use of information by departments, and large increases in storage facility and administration costs.
Recently, as a measure against increases in data volume, storage consolidation technology is promising. One such embodiment is, for example, a file server that can be shared by multiple clients and is used at a data center. Here, a client is a user of an information technology (IT) system. Furthermore, since a client is called a tenant, a file server that can be shared may be called a multi-tenant file server.
A multi-tenant file server assumes utilization by an enormous number of tenants. Naturally, the number of users belonging to each tenant is enormous. Therefore, in a system using a multi-tenant file server, a huge storage capacity, high response performance, and throughput are demanded. As an example of configuration of a system that satisfies these conditions, a system uses data servers each having a different function, such as a system consisting of multiple data servers and a meta server.
A data server is a file server that controls the input/output (I/O) process of each file handled by each tenant. A meta server is a file server that manages metadata, which expresses the file identifying information (file name, file type, storage location information, etc.). A file server group having a file server function configured by multiple data servers and a meta server is called a cluster file server. If configuration is like a cluster file server, where data servers and the meta server are independent, the I/O processing load for each file can be distributed among the multiple data servers.
In a system that uses a cluster file server, system administration that reduces TCO is expected. For example, each of the data servers and the meta server as well are shared by multiple tenants, facilitating cost reductions by increasing the utilization efficiency of computing resources.
In the operation of a cluster file server, as a data protection, periodic backup processing is required. Technology adopted for implementing the backup processing is often a snapshot function, but disk images collected by the snapshot function cannot be utilized unless the images are in a consistent state.
Here, if a snapshot is collected, each file server performs a quiesce operation. In the quiesce operation, at each file server, new accesses are queued until either the write-backs of the dirty cache to disk, or the snapshot collection is completed. By performing the quiesce operation, each file server can preserve the consistency of the disk image collected as a snapshot. (For examples of conventional arts, refer to Japanese Laid-Open Patent Publication Nos. H11-120056, 2006-146904, 2004-38929, and 2009-53955.)
However, as described, with the multi-tenant servers, to accommodate an enormous number of users, the number of data servers and the cache capacity provided thereto also becomes huge. In other words, the amount of data that is to be subject to backup processing at each file server is also huge.
Further, when a snapshot is collected, all of the cache data held by all of the data servers is simply written back to a disk. Therefore, a problem arises in that at each file server, an enormous amount of overhead occurs related to the collection of a series of snapshots, including the quiesce operation.
Additionally, depending on the data type and system operation, among the tenants using the file servers, there may be some tenants who do not require snapshot collection. For tenants who do not require snapshot collection, periodic snapshot collection is meaningless. Furthermore, write-backs of the cache data to disk during the quiesce operation pose a problem of interfering with performance for tenants that do not require snapshot collection.